1. Field of the Invention
The invention relates to a novel formulation of thickeners, based in particular on polyurethane, for thickening aqueous preparations for, for example, coating systems.
2. Description of the Related Art
A multiplicity of polyurethane-based associative thickeners is known. They are linear or branched, generally nonionic surfactants having discrete hydrophilic and hydrophobic domains. Their typical structures, their preparation and use are described inter alia in U.S. Pat. No. 4,155,892 or U.S. Pat. No. 4,079,028.
The polyurethane thickeners are prepared from                (a) at least one water-soluble polyetherpolyol,        (b) at least one water-insoluble organic polyisocyanate,        (c) at least one monofunctional hydrophobic organic compound selected from compounds having a hydrogen atom which is active toward isocyanates and from organic monoisocyanates, and        (d) at least one polyfunctional alcohol or polyfunctional ether alcohol.        
EP-A-307 775 describes water-dispersible, modified polyurethane thickeners which are prepared from                (a) a polyisocyanate,        (b) a polyetherpolyol,        (c) a modifier having at least 2 active hydrogen atoms and at least one hydrophobic group, said modifier containing no groups which can react with polyisocyanate or with the polyetherpolyol, and        (d) an endcapper, such as alkoxylated alcohols, for example.        
U.S. Pat. No. 4,327,008 describes star-shaped PU thickeners which are reaction products of                (a) polyetherdiols,        (b) higher polyfunctional (>=3) polyetherpolyols or isocyanates,        (c) a diisocyanate,        (d) based on the polyetherdiol, from 37 to 175 mol percent of water, and        (e) an endcapping monool or monoisocyanate.        
Further polyurethane thickeners are described in EP-A-0 31 777, EP-A-495 373, U.S. Pat. No. 4,499,233, U.S. Pat. No. 4,426,485, DE-A-4 101 239, and U.S. Pat. No. 5,023,309.
Polyurethane thickeners of the type stated and their preparations are suitable auxiliaries for adjusting rheological properties of aqueous coating systems such as automotive finishes and industrial coatings, plasters and masonry paints, printing inks, pigment pastes, filler dispersions or cosmetic preparations: U.S. Pat. No. 4,155,892, U.S. Pat. No. 4,079,028, EP-A-0 307 775, U.S. Pat. No. 4,327,008, EP-A-0 031 777, EP-A-0 495 373, U.S. Pat. No. 4,499,233, U.S. Pat. No. 4,426,485, DE-A-41 01 239, and U.S. Pat. No. 5,023,309.
All of these polyurethanes described in these prior publications, which are herein incorporated by reference, share the feature that hydrophilic segments are present in an amount of at least 50% by weight, with not more than 10% by weight of hydrophobic segments and urethane groups. Hydrophilic segments here are, in particular, high molecular mass polyether chains, composed in particular of ethylene oxide polymers. Hydrophobic segments are, in particular, hydrocarbon chains with at least six carbon atoms.
The skilled worker is aware that effective thickeners can only be obtained if the hydrophilic polyether segments have a molecular weight of at least 6,000 g/mol, are composed almost exclusively of polyethylene oxide, the hydrophobic segments have at least 12 carbon atoms, and hydrophilic and hydrophobic segments are in a balanced weight ratio of 92-97% to 8-3%.
In addition, these polyurethanes ought to have a very low inherent viscosity in order that, where appropriate in the form of very highly concentrated solutions, they can be processed and handled without problems. This requirement prevents, for example, the obvious preparation of long hydrophilic segments by chain-extending reaction of comparatively low molecular mass polyetherdiols with diisocyanates, since the greater number of urethane groups this entails would result in an unwanted increase in the inherent viscosity.
Recently a large number of experiments have been undertaken to lower the inherent viscosity of thickeners. The skilled worker is aware, however, that simply reducing molecular weight goes hand in hand with a deterioration in the thickener's effectiveness. Another possibility is based on the addition of typical emulsifier structures (DE-A-19 600 467), including in particular acetylenediol derivatives (EP-A-0 618 243). Also described is the additional admixing of diesters (DE-A-19 644 933). In U.S. Pat. No. 5,597,406 formulation is carried out using a diol or a diol monoalkyl ether, a salt of a sulfated nonylphenol ethoxylate, organic phosphoric esters, and nonionic surfactants.
These methods have the drawback to start with that these surfactants must be used at high concentrations in order to achieve a satisfactory reduction in the thickener's inherent viscosity. In addition they are accompanied by a range of further drawbacks, such as the stabilization of foam in the thickener preparations that is caused by these surfactants, and in particular aqueous systems in which they are present, such as dispersion-based masonry paints, for example.
Moreover, this introduction of surfactant undesirably impairs the water stability and weathering stability of coating systems and also, in the case of masonry paints, their abrasion resistance.
The high-performance alkylphenol ethoxylates have come under criticism on ecotoxicological grounds and their use is already legislatively regulated.
A further common method, known from the literature, of reducing the inherent viscosity of aqueous polyurethane solutions is to add water-soluble or water-miscible solvents such as alcohols or glycol derivatives, for example. The critical drawback of this approach, however, is that it involves the introduction of solvents into environmentally compatible coating systems to an undesirable extent, running counter to the concept of VOC reduction.
It is known that the problems depicted occur to an increased extent in connection with branched polyurethane thickeners of extreme pseudoplasticity.
Applications of styrene oxide block copolymers are still largely unknown in the printing inks and coatings industries. Known examples include (DE-A-19 806 964) polystyrene oxide-(block)-polyalkylene oxide copolymers, which, starting from a monofunctional starter alcohol, are reacted by sequential addition reaction of at least 2 mol of styrene oxide and an alkylene oxide, with subsequent phosphorylation to form the corresponding phosphoric esters.
These block copolymers are used particularly in their neutralized form, as the sodium or potassium salt, for example, as dispersants. Because of the electrolyte content and the high hygroscopicity, however, it is inappropriate to use them in larger amounts to formulate inks and paints. Where these compounds enter the environment, their lack of rapid biodegradability, a consequence of the relatively long styrene oxide blocks present, is a drawback.